It's a bit 'out there'. I have a bird name. Once dated a guy named Gabriel who made a point of the fact that he was named after an angel, while I was named for a common bird...I dumped him. At least your bird is uncommon.

I'm all for unusual names and unusual word names are my guilty pleasures. Kestrel sounds boyish for some reason to me, but as it's not really used, there's no reason it can't be a girl name. I think it has sort of a hard sound, but Kess (so similar to Cass) or Kessy are so cute.

I usually dislike names like this that aren't really established as names, per se, but this one I like. And the nn Kes makes me think girl (after the character on Star Trek Voyager - yeah, I'm a nerd).